Operating electric grids with more intermittent renewable energy sources, wind and solar, do not come without challenges. As discussed in my previous blog, The Importance of Strong and Nice Neighbors, the experiences from Denmark and Germany illustrates the value of strong electric ties with neighboring systems.
Generally the bigger the electric system the easier it is to balance intermittent renewable energy. A larger system benefits from more fuel diversity as well as more different demand patterns.
Without the neighboring systems Denmark would not be able to get 30 % of its electric energy from wind. Denmark on its own is too small to absorb the fluctuations and maintain reliability of the system.
Even Germany with a 15 times bigger system than Denmark but “only” 13 % of the energy from wind and solar is highly dependent on export/import to and from the neighboring systems.
How do we compare in the USA? Let’s take a look at three states: Hawaii, Texas and California.
Hawaii has no neighboring electric systems. In fact each of the Hawaii islands is on its own. There are no interconnections between the respective electric grids. Presently (2013) over 70 % of all power generated is by oil fired power plants. Just about 12 % is renewable energy and substantial portion of that is geothermal power on the main island. In 2011 solar provided only 1 % of Hawaii’s electricity. However, solar is growing. In 2012 grid-connected solar reached 200 MW. Hawaii Electric (HECO) got so concerned with the grid stability on Oahu due to the growth in rooftop residential PV, that in October last year they required homeowners to have HECO’s approval before installing a PV system.
Hawaii has set a very ambitious goal of reaching 40 % renewables by 2030. The State has potentially a large amount of renewable energy, not only wind and solar but also geothermal and biomass. The challenge is to bring it to the load and to keep the system(s) stable. In a study 2009 by NREL (Department of Energy/National Renewable Energy Laboratory) focused on Oahu, the island with the biggest load, the main conclusion was that it would require a radical expansion of the transmission system, including connecting all the islands. Technically it is probably feasible, but the costs would be very high. Since Hawaii has already the highest retail electricity prices in the US it seems unlikely that such a massive expansion of the transmission system is economically viable. Instead one has to look for more cost efficient solutions, which probably will have less extensive expansion of the transmission interconnections but more of energy storage as well as more base-load biomass on each island.
Texas is an interesting state for two reasons. With over 12 GW (2012) it is along with California a leader in installed wind power in the US. Electrically Texas has a “stand-alone” and independent (not synchronized with neighboring systems) grid. It is a large system, but there are only 5 HVDC (High Voltage Direct Current) ties totaling 1.1 GW with the surrounding systems. Consequently, by and large Texas must balance the intermittent generation with other resources within the state.
In 2013 about 10 % of the electric power delivered was by renewable energy sources, predominantly wind. The bulk of the power production was from natural gas, about 45 % and coal, about 35 %. The high portion of natural gas fired generation makes balancing the intermittent wind easier. Nevertheless Texas has been facing the challenge of having the wind in the western parts of the state, while most of the load is in the eastern parts of the state. In January 2012 ERCOT (Electricity Reliability Council of Texas) announced plans for $8.7 B during 5 years for new and upgraded transmission lines.
To further expand with more wind and now also more solar it would help if Texas had stronger electric ties with the neighboring systems. One of the more intriguing, technically and economically, opportunities could be be to connect with Tres Amigas SuperStation, which is under development.
Tres Amigas, which is located in New Mexico, targets to connect the three unsynchronized US grids: Eastern Interconnect, Western Interconnect and Texas. The technical concept is a three-way back to back HVDC station using HVDC converters and superconductive DC cables. The market concept is to provide an Exchange with spot as well as forward markets. Tres Amigas phase one is to connect the Western Interconnect with the Eastern Interconnect with a capacity of 750 MW. Phase two would be to expand with another 750 MW and connecting Texas. Tres Amigas could be further expanded to 5000 MW, but it will also require strengthening of associated transmission systems.
California is in terms of renewable energy the leading state in the US. About 30 % of the electric energy consumed comes from renewable sources including “large hydro”. Excluding the latter the renewable sources have on a daily basis fluctuated between 6 % and 26 % of the consumed power (during January 2012 – September 2013.)
In terms of solar California has the largest amount of installed capacity as well as the fastest growth in the nation of utility-scale solar. On January 2 2013 it hit a record of 1235 MW. One year later it reached a new record of 3048 MW! In addition it is estimated that behind the meter close to 2000 MW of rooftop solar has been installed.
California has strong ties to surrounding systems. It has enabled California to import about 30 % of its electric energy as well as helped balancing the large amounts of intermittent wind and solar. Most of the imported power has historically been hydro from Columbia River and thermal power from Arizona and Utah, but going forward also wind and solar will be part of the imports.
California has set the goal for renewables, excluding “large hydro”, from present 18 % to 33 % by 2020. With such high ambitions for renewable energy as well as ‘replacing the 2.1 GW of power from unit 2 and 3 of San Onofre nuclear generating station (SONG), California is, no surprise, not only strengthening the California transmission system but also working with other States to expand the electric ties with the surrounding systems.
In conclusion managing more intermittent renewable energy, wind and solar, one needs a toolbox to manage the fluctuations and balance the electric system. Flexible base-load generation, gas turbine “peakers”, demand response and storage all helps. However, without strong transmission systems, including strong ties with neighboring systems there is an upper limit how much intermittent power a system reliably can handle.